Many transformers and other electrical equipments use cellulose as an insulating element in their manufacture. The dielectric properties of the cellulose along with its abundance, its durability, and the facility in its manipulation during the manufacture processes, they become an element ideal for the manufacturing of electric. It is common to find equipment that use dielectric fluids, moreover the cellulose, as booster of the dielectric capacity of the cellulose, moreover they serve as mean for transfer of heat to the outside, protecting to the equipment of damage due to the exposure to high temperatures of the electric material. Nevertheless, despite of the multiples advantages of this material, it is well known for more than half century that its aging involves to the loss of some of its fundamental properties, among them its dielectric stiffness. Because of this reason, it is fundamental to do all possible to avoid the degradation of this component and to avoid possible problems with the operation of the equipments related to failures of dielectric material.
Power electrical transformers are devices that modify the voltage in the electrical circuits for power transmission, its power is high, so that also, the ability of current conduction is rather high. Due to that the power electric transformer use very high voltages, insulating systems based on cellulose (paper, carton, wood) are used, as well as a dielectric fluid that serves as mean of cooling, it is necessary that the content of humidity be low inside the power electric transformers to avoid to impact in the dielectric properties of these components, not only in the first periods of operation, but along its operating life. These appliances are disarmed in the plant prior to shipment and subsequently armed and reconnected once they arrive to the substation where they will be used. For its reconnection and inspection in the interior, specialized personnel get inside the tank of a power electric transformer, but is required for, security reasons, that a totally open registry be maintained, which favors there has exchange between the atmospheric air and the humidity that this contain, and the water to be placed in the surface of the conductors and its isolations. Once the transformer is inspected and internal connections made and before filling it with dielectric oil for its powering; it is required to remove the moisture in the air and deposited it on the surface of the isolations. For the drying process described above there is a practical and conventional method, which takes place by extracting the air and lowering the pressure inside the tank of the power electric transformer by means of a vacuum pump, but not before ensuring the tightness in the tank. The connection between vacuum pump and the closed tank is carried out by means of a hose or pipeline. Such a process can last several days depending on the humidity that was introduced and size or volume of the tank. There are also tests to determine with certainty the level of humidity of the transformer. Most of these tests are related to the calculation of the pressure inside the tank, since the pressure inside the tank, it is a result of the steam pressure that has the humidity of the interior. Units of measurement of pressure that are commonly used are the millimeters of mercury, with the understanding that the start of the extraction process, the system has an atmospheric pressure that is about 760 mm of mercury at sea level, either an atmosphere.
During installation and start-up of a power transformer, the drying process is one of the activities that takes longer to be completed, moreover from the risks of a reprocessing and dead times that could exist in the determination of the humidity test as validation test to conclude the drying process.
Because of the aforementioned, the monitoring of the efficiency of the process of extraction humidity or drying has a great value. From time to time said monitoring is done by isolating (closing an intermediate valve between the tank of the transformer and the vacuum pump) the tank and measuring as the pressure grows in the same, that is to negative pressure (but without extraction). In this method, the depression (increase of pressure in a closed container) according to the volume of the tank during half an hour. If the increase is significant, then it is likely that an air entry exists at any part of the transformer or that the high pressure of steam is causing that the pressure within the tank increase in a fast form.
There are conventional methods used by most of the installers of power electric transformer to determine the moisture in the transformer and that define the termination of the drying process.
The most used method used to measure moisture content inside the power electric transformer is by injecting air or extra-dry nitrogen, once the vacuum process is cut, it should fall back to levels of 0.5 mm of Hg of pressure.
Subsequently, the air injected is allowed to mix with the moisture remaining on the surface inside the tank for a time of between 12 and 24 hours. Later, the dewpoint temperature is measured (which refers to the temperature to which the moisture contained in a gas starts to condense) in a sample (nitrogen or extra dry air)that is pressuring to the transformer, to determine steam pressure of such gas through adjusted graphics for this measurement (Piper or Oomen charts). A lower dewpoint temperature implies a smaller humidity in the gas. Now, once the value of this variable is obtained (steam pressure), with another table and with the approximate temperature of the isolations and the value of steam pressure, the residual humidity is calculated. This process has the inconvenience of requiring around 1.5 days to calculate the moisture content.
The other process to determine the content of humidity in the power transformers is via the measurement of the flow of water steam that is extracted of the same one, nevertheless the flow that has interests at the end of the process of drying. Conventionally a parameter has been established for maximum flow according to the content of isolations (kilograms of wood, carton and paper) and in this manner, to calculate the humidity in the transformer. To account such water flow, the steam that flows during the extraction is collected via the steam condensation of the same and subsequently the melting of the ice (condensed) for its measurement in a calibrated container. The mechanism to get the humidity is a surface at very low temperatures (below −45° C.), since the quantity of water in the air that flows is extremely low which, makes necessary to utilize cryogenics temperatures and therefore to condense the steam. This system is called cold trap, in some cases it uses carbon dioxide in the form of dry ice and acetone for cooling a surface homogeneously or with liquid nitrogen. The established procedure indicates that the condensate in order to be considered representative, it must have a collection of at least 4 hours of continuous flow.
In either both of the most common methods there are disadvantages involving dead times and, as well as, low repeatability or reproducibility. In the case of the measurement which requires the injection of an extra dry gas, the major constraint is the dead time in which they are incurred from when the gas is injected (which you can be between 2 and 4 hours depending of the size of the unit) moreover the time that should be expected to allow the extra dry air reaches a certain balanced with the moisture of the interior (which regularly requires more than 12 hours). Finally, within these same dead times and once it is within the allowed level of moisture, then starts another vacuum process for filling (taking about 2 to 4 hours). It should be noted that the skill persons recommend to perform this test at temperatures above 10° Celsius and this is because the behavior become rarefy as the temperature lowers, (it is not strange that in the field to reach temperatures lower than 10 Celsius in the transformer during the night), which delays the results for more hours. Therefore, since this is not a measure instrument for continuous measurement, is considering about 24 hours to know the conditions of humidity inside the apparatus and to be able to fill the unit with oil.
In the measurement method with water collection via a cold trap, a main disadvantage is the required preset, because of the very low amounts of water, it is important that the period is at least 4 hours to collect a sufficient amount of moisture, and the other disadvantage is that the cold trap is not regulated, so that the variation in design (it could collect a different quantity of water from the cold surface where water particles contact) in addition that the resulting fluid is a mixture of transformer oil and water (slurry state), which sometimes makes it difficult to measure humidity.